Passing String to a function that takes uint8_t in C

Question

I want to send a string over UART but the function that sends data over UART accepts uint8_t as an argument.

The function prototype: UART2_Write(uint8_t txData);

I wish to send an 8-character long string such as "12345678" and I have tried the following ways:

#define UID ("12345678")
UART2_Write(UID);

char UID[8] = "12345678";
UART2_Write(UID);

const char *UID = "12345678";
UART2_Write(UID);

Unfortunately, none of the above-mentioned methods yielded a successful result.

If there is a way to send a string in this situation, kindly let me know.

Thanks

Sounds like you need to loop over all the characters in your string, calling UART2_Write multiple times, once for each character. — Steve Summit
– Steve Summit, Commented Oct 5, 2022 at 18:42

Willis Hershey · Accepted Answer · 2022-10-05 21:05:29Z

2

Sending an entire 8-byte string in a single UART serial communication is not strictly possible, since UART sends data in blocks of 5-9 bits at a time. Based on the input parameter type of UART2_Write(), this transmission appears to be done in 8-bit mode. You must send each of the bytes in the string individually.

This can be done by looping over the characters in the string.

for instance:

for(size_t i = 0; UID[i] != '\0'; ++i)
  UART2_Write(UID[i]);

then, if the serial receiver is only expecting 8-bytes you're done, otherwise you may need to send a terminating character, like 0, which corresponds to NULL in ASCII, or perhaps you may want to send EOT (end of transmission), which corresponds to 0x4 in ASCII. To find out what you should send to signal that the transmission is over, you may need to read documentation on the device being communicated with via UART.

edited Oct 5, 2022 at 21:05

answered Oct 5, 2022 at 19:28

Willis Hershey

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7 Comments

Rafay Shams Over a year ago

I see. On the receiver side, will the string appear as one continuous string? Or will it be one character at a time?

Willis Hershey Over a year ago

The receiver will get the string 1-byte at a time, but if it's expecting a string, that is how it will expect to receive it and it shouldn't be a problem

Willis Hershey Over a year ago

Even modern internet connections work by sending data 1-byte at a time. This is just how digital connections tend to work

dimich Over a year ago

strlen() is redundant here. size_t i = 0; do UART2_Write(UID[i]) while(UID[i++]); Or const char *s = UID; do UART2_Write(*s) while(*s++); - if null terminator also must be sent.

Willis Hershey Over a year ago

This is true, but any compiler worth its weight in parmesan cheese is going to optimize strlen(<macro that expands to a string>) into an integer. It is definitely worth noting that neither strlen() nor the while loop @dimich suggests will work for the second example where the char[] is only 8-bytes long and therefore not null-terminated

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